1. Field of the Invention
The present invention relates to a PWM circuit according to a multi-rate method.
2. Description of the Related Art
Conventionally, a PWM circuit that controls electric power supplied to a load by varying a duty ratio of a pulse signal has been in use for controlling speed of a motor, dimming of lighting equipment, and the like. When a high-level period of such a PWM circuit is lengthened, the electric power supplied to the motor and the lighting equipment increases, resulting in an increase in a rotational speed of the motor and brightness of the lighting. On the contrary, when the high-level period thereof is shortened, the electric power supplied to the motor and the lighting equipment decreases, resulting in a decrease in the rotational speed of the motor and brightness of the lighting (see, for example, Japanese Unexamined Patent Application Publication No. Hei 07-183779).
Variation of the high-level period is generally done by changing the number of reference clocks counted in this period, therefore, a length of the high-level period can only be changed in increments/decrements of a reference clock. For example, assuming that the number of the reference clocks to generate one cycle of output of the PWM circuit is 10000 clocks, when the high-level period is increased by one clock, an incremental ratio of the high-level period will be 1/10000. However, the incremental ratio cannot be 1/10000 or less. Heightening the frequency of the reference clocks makes the width of one clock shorter, enabling high-precision variation of the period. But, in order to achieve that, a counter or the like with a high operational speed is needed, creating an operational limit and increasing cost. In view of this problem, there is a PWM circuit according to a multi-rate method in common use which enables precise variation by one reference clock or less.
For example, a conventional PWM circuit according to the multi-rate method controls electric power supplied to a motor or the like with precision of one reference clock or less by, for example, increasing the length of the high-level period by one clock once in several cycles of a pulse signal outputted from the PWM circuit while keeping a cycle of the pulse signal unchanged. For example, assuming that the number of the reference clocks for generating one cycle of the output of the PWM circuit is 10000 clocks, when the length of the high-level period is increased once by one clock in two cycles, an incremental ratio of the high-level period is 1/20000. Likewise, when it is increased once by one clock in four cycles, the incremental ratio of the high-level period can be 1/40000, so that high-precision control is enabled irrespective of the frequency of the reference clocks.
The conventional PWM circuit according to the multi-rate method can perform power control with high precision of one clock or less by increasing the length of the high-level period once in several cycles of the PWM output. However, since the high-level period thereof contains increased cycles and non-increased cycles both, jitter occurs in a signal waveform of the PWM output. With the jitter in the signal waveform of the PWM output, a signal given to the load of a motor or lighting equipment will include a distortion component therein, which problematically affects the operation of the equipment.
A possible solution to remove such distortion is to use a filter or the like, but this also involves a problem of increased circuit scale and cost.